We have performed studies on the short term modulation of the regulation of serotonin synthesis, and long term studies of the site and mechanism of toxicity of P-chloroamphetamine and fenfluramine in discrete rat brain nuclei. In the acute experiments, changes in the tryptophan hydroxylase activity after treatments which modify tryptophan concentration were measured using high pressure liquid chromatography. In the chronic studies, we measured serotonin using mass spectrometry and tryptophan hydroxylase as specific markers for serotonergic function.